1. Field of the Invention
The present invention generally relates to data processing methods, circuits, and apparatuses, and more particularly to a data processing method that is useful in recording a multi-level signal on and reproducing a multi-level signal from an information recording medium such as an optical disk (for instance, a CD-R disk, a CD-RW disk, or a DVD disk), a magneto-optical disk, a magnetic disk, or a magnetic tape, or in transmitting and receiving a multi-level signal through a wireless or wire transmission path, a data processing circuit mounted in an information storage device or a communication device to which the data processing method is applied, and a data processing apparatus, such as an information storage device or a communication device, to which the data processing method is applied.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 8-124167 discloses an optical information recording and reproduction method and apparatus that, in processing a multi-level information signal reproduced from a recording medium, detect data with an excellent eye opening obtained by performing equalization (waveform equalization) to eliminate intersymbol interference between adjacent data.
Normally, it is a premise in information recording and reproduction to obtain an excellent eye opening by performing signal processing such as waveform equalization as in the above-mentioned prior art technology. As recording density becomes higher, however, intersymbol interference increases, so that waveform equalization reaches its limit. As a result, an excellent eye opening cannot be obtained, resulting in the problem of incorrect detection of data at the time of data reproduction.
According to a conventional multi-level data processing method, test data for all the combinations of a plurality of (three) succeeding multi-level data are recorded on an information recording medium, and a table recording the reproduced signal values of the test data is created at the time of reproducing multi-level data so that multi-level data whose error between its reproduced signal values and the signal values recorded in the table is minimum is output as reproduced multi-level data.
Further, according to another conventional method of determining multi-level data using such a table, after determining multi-level data on both ends of the three succeeding multi-level data by using fixed thresholds, multi-level data that minimizes an error between its center reproduced signal value and a signal value in the table is output as reproduced multi-level data. Thereby, the accuracy of determination is increased with the compact table.
According to yet another conventional method of determining multi-level data, with a plurality of multi-level data being treated as a set, binary data is converted to multi-level data by arranging the binary data directly in upper-side bits of the multi-level data and arranging data in the remaining lower-side bits of the multi-level data in accordance with a predetermined conversion rule. At the time of reproducing the multi-level data, since the lower-side bits thereof are likely to be determined wrongly, the multi-level data is determined using the fact that the lower-side bits of the multi-level data comply with the predetermined rule.
According to this method of processing multi-level data, at the time of recording or transmitting multi-level data of n bits per symbol (n is an integer satisfying n≧2), with m symbols (m is an integer satisfying m≧2) being a set, binary data is arranged in the upper-side (n−1)×m bits ((n−1) bits×m symbols) of the m symbols and binary data obtained by converting m−1 bits to m bits (corresponding to the predetermined conversion rule) is arranged in the lower-side bits of the m symbols. Thereby, one bit of redundancy data is generated due to the predetermined conversion rule, while the accuracy in multi-level data determination is increased.
Further, the data is arranged in the lower-side bits of the multi-level data in accordance with the predetermined conversion rule in the above-described processing, which is effective when a data error is limited to the lower-side bits at the time of reproducing multi-level data. However, if the data error occurs beyond the lower-side bits, the multi-level data is wrongly determined. Therefore, there has been proposed a multi-level data processing method by which data is arranged in the lower-side k bits of multi-level data in accordance with a predetermined conversion rule.
According to this multi-level data processing method, at the time of recording or transmitting multi-level data of n bits per symbol (n is an integer satisfying n≧2), with m symbols (m is an integer satisfying m≧2) being a set, binary data is arranged in the upper-side (n−k)×m bits ((n−k) bits×m symbols) (k is an integer satisfying n>k≧1) of the m symbols and binary data obtained by converting m×k−1 bits to m×k bits (corresponding to the predetermined conversion rule) is arranged in the lower-side m×k bits (lower-side k bits×m symbols) of the m symbols. By thus arranging the data in the lower-side k bits of the m symbols, accuracy in multi-level data determination is increased even if characteristically, errors are likely to occur in the transmission path of data, such as an information recording medium (an optical disk, for instance) or a communication path.
The above-described multi-level data processing methods are kinds of pattern recognition methods that recognize intersymbol interference that occurs at the time of recording successive multi-level data sets as interrelationships among the successive multi-level data sets. Therefore, when the multi-level data can take numerous values or a reproduction signal includes much noise, patterns in a table become less distinctive so that the multi-level data is likely to be determined wrongly.
Further, at the time of reproducing multi-level data, first, a reproduction signal is subjected to waveform equalization to eliminate intersymbol interference therefrom, and then candidate multi-level data are output based on fixed thresholds. Thereafter, candidate multi-level data whose lower-side bit data complies with a predetermined conversion rule are selected, so that candidate multi-level data having a minimum error is output as reproduced multi-level data. Therefore, intersymbol interference remains slightly after the waveform equalization. This may cause an error at the time of outputting candidate multi-level data based on fixed thresholds, leading to the problem that the final reproduced multi-level data is wrongly determined.